Genetically modified yeasts and fermentation processes using genetically modified yeasts

ABSTRACT

The present invention relates to a genetically engineered yeast capable of manufacturing a fermentation product using sucrose as a fermentation substrate, and fermentation processes using such a yeast. The yeast has an exogenous invertase gene and has a deletion or disruption of the PDC activity gene. Accordingly, the yeast is useful for manufacturing fermentation products other than ethanol from fermentation substrates containing sucrose.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No. 62/259,531, filed Nov. 24, 2015, which is hereby incorporated by reference in its entirety.

SEQUENCE LISTING

The entire contents of the ASCII text file entitled “N003 16SEQID2.txt,” created on Nov. 22, 2016, and having a size of 82 kilobytes is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

Industrial yeast fermentation processes often use glucose-based substrates in regions of the world where such substrates are readily available. For example, glucose syrup made from corn starch is commonly used in fermentation processes in the United States. However, in some regions, sucrose substrates are more readily available and/or more economical for use in fermentation processes, or it is desirable to use such sucrose substrates as a supplement to glucose substrates.

SUMMARY OF THE INVENTION

Described herein are genetically engineered yeasts useful for manufacturing fermentation products and fermentation processes based on the use of such yeasts. In one aspect, the present invention relates to engineering yeasts to use sucrose as a fermentation substrate from host yeasts that are incapable of using sucrose or are inefficient at using sucrose as a fermentation substrate. Accordingly, the yeasts of the present invention have a functional invertase gene. In one aspect, the yeasts are engineered to include promoters that are associated with an optimized expression of invertase.

In one aspect, the genetically engineered yeast comprises a yeast capable of producing a fermentation product at a production rate of at least 1.0 grams/liter-hour (g L⁻¹ h⁻¹), wherein the genetically engineered yeast has a functional invertase gene and has a deletion or disruption of the pyruvate decarboxylase (PDC) gene. In some embodiments, the yeast is capable of producing a fermentation product at a fermentation production rate of at least 1.5 g L⁻¹ h⁻¹ or at least 2.0 g L⁻¹ h⁻¹. In some embodiments, the yeast is capable of producing a fermentation product at a pathway fermentation yield of at least 55 percent, at least 65 percent, at least 70 percent, or at least 75 percent. In some embodiments, the yeast is capable of producing a fermentation product at a final titer of at least 30 g/liter, at least 80 g/liter, or at least 100 g/liter. In some embodiments, the yeast has a ratio of invertase activity to glucose capacity of less than 95, less than 30, or less than 20. In some embodiments, the yeast has a ratio of invertase activity to glucose capacity of at least 0.95 or at least 10. In some embodiments, the yeast has a ratio of invertase activity to glucose capacity of at least 2.5, 3, or 5.

In another aspect, the genetically engineered yeast capable of manufacturing a fermentation product is a yeast of the I. orientalis/P. fermentans clade having a gene encoding a functional invertase. In one embodiment, such a yeast is PDC-negative. In one embodiment, the yeast is I. orientalis.

In some embodiments, the yeast is Crabtree-negative. In some embodiments, the functional invertase gene is selected from the group consisting of SEQ ID NO: 6; SEQ ID NO: 15; SEQ ID NO: 16; and SEQ ID NO: 17. In some embodiments, the yeast includes an exogenous or artificial promoter for the functional invertase gene. In some embodiments, the promoter is selected from the group consisting of Pyruvate decarboxylase, Glyceraldehyde-3-phosphate dehydrogenase, Translational elongation factor, Transaldolase, RPL16B, 3-phosphoglycerate kinase, and Enolase. In some embodiments, the yeast is capable of manufacturing any of the following fermentation products: lactic acid, citric acid, malonic acid, hydroxy butyric acid, adipic acid, lysine, keto-glutaric acid, glutaric acid, 3-hydroxy-proprionic acid, succinic acid, malic acid, fumaric acid, itaconic acid, muconic acid, methacrylic acid, or acetic acid, or any derivatives thereof, any salts thereof, or any combinations thereof.

In one aspect, the process is a process for manufacturing a fermentation product comprising fermenting a substrate using any of the genetically engineered yeasts described herein. In one aspect, the process is a process for manufacturing a fermentation product comprising: fermenting a substrate using a yeast, wherein the substrate includes sucrose and the yeast includes an exogenous invertase gene.

In some embodiments, the process is microaerobic. In some embodiments, the volumetric oxygen uptake rate (OUR) is 0.5 to 40 mmol 02/(L-h), 1 to 30 mmol 02/(L-h), 3 to 20 mmol 02/(L-h), or 5 to 16 mmol 02/(L-h). In some embodiments, the specific OUR is 0.2 to 13 mmol 02/(g cell dry weight-h), 0.3 to 10 mmol 02/(g cell dry weight-h), 1 to 7 mmol 02/(g cell dry weight-h), or 2 to 6 mmol 02/(g cell dry weight-h).

In some embodiments, the fermentation cell concentration of the process is 1 to 10 g cell dry weight/L, 2 to 8 g cell dry weight/L, or 2.5 to 6 g cell dry weight/L. In some embodiments, the pitch density of the process is 0.05 to 5 g cell dry weight/L, 0.05 to 4 g cell dry weight/L, or 0.05 to 2 g cell dry weight/L. In some embodiments, the fermentation temperature is in the range of 25 to 45° C., in the range of 20 to 40° C., or in the range of 33 to 38° C. In some embodiments, the fermentation substrate of the process comprises sucrose, glucose, hydrozylates of starch, xylose, lignocellulosic hydrozylates, or any mixture or any combination thereof.

In some embodiments, the process has a ratio of invertase activity to glucose consumption rate of less than 95, of less than 30, or of less than 20. In some embodiments, the process has a ratio of invertase activity to glucose consumption rate of at least 0.95 or at least 10. In some embodiments, the fermentation yield of the process is at least 55 percent, at least 65 percent, at least 70 percent, or at least 75 percent. In some embodiments, the final titer is at least 30 g/liter, at least 80 g/liter, or at least 100 g/liter. In some embodiments, the fermentation product of the process is lactic acid, citric acid, malonic acid, hydroxy butyric acid, adipic acid, lysine, keto-glutaric acid, glutaric acid, 3-hydroxy-proprionic acid, succinic acid, malic acid, fumaric acid, itaconic acid, muconic acid, methacrylic acid, or acetic acid, or any derivatives thereof, any salts thereof, or any combinations thereof.

In some embodiments, the invertase gene in the yeast is an integrated functional exogenous invertase gene. In some embodiments, the invertase activity of the yeast or the yeast in the process is at least 1, 2, 2.5, 3, 4, 5, 6, 7, 8, or 9 (g glucose released/(g CDW*h)). In some embodiments, the invertase activity of the yeast or the yeast in the process is less than 10, 15, 20, 30, 40, or 50 (g glucose released/(g CDW*h). In some embodiments, the invertase activity of the yeast or process is in the range of about 2.5-50, 5-30, or 5-20 (g glucose released/(g CDW*h)). In some embodiments, the ratio of invertase activity to glucose consumption rate (or glucose capacity) of the yeast or process is in the range of about 0.5 to 25 or 1 to 20.

It is also to be understood that the elements or aspects of any embodiment of the processes, methods, or compositions described above can be applied to any other embodiment, as would be understood by a person skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the invention will be better understood when read in conjunction with the appended drawings. It should be understood, however, that the invention is not limited to the precise arrangements and instrumentalities of the embodiments shown in the drawings.

FIG. 1 is a graph showing sucrose (squares), glucose (diamonds), fructose (triangles), and succinate (circles) titers for an exemplary fermentation process using yeast strain 1-8.

FIG. 2 is a graph showing glucose (solid lines) and succinate (dashed lines) titer for exemplary fermentation processes using strain 1-5 (circles) and strain 1-8 (squares).

FIG. 3 is a graph showing glucose (solid lines) and succinate (dashed lines) titer for an exemplary fermentation process using strain 1-1 (squares), strain 1-2 (circles) and strain 1-4 (triangles)

DETAILED DESCRIPTION

It is to be understood that the figures and descriptions of the present invention provided herein have been simplified to illustrate elements that are relevant for a clear understanding of the present invention, while eliminating other elements found in the related field(s) of art. Those of ordinary skill in the art would recognize that other elements or steps may be desirable or required in implementing the present invention. However, because such elements or steps are well known in the art or do not facilitate a better understanding of the present invention, a discussion of such elements or steps is not provided herein.

Definitions

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one skilled in the art to which this invention belongs. As used herein, each of the following terms has the meaning associated with it as defined in this section.

Fermentation Process Definitions

As used herein, “inoculation” is defined as the point in time wherein a microorganism capable of producing a fermentation product is introduced into a fermentation medium. This is a term that is well known to those skilled in the art.

As used herein, “end of fermentation” is defined as the point in time where a fermentation process meets a predetermined criteria. The predetermined criteria can include any of the following: a predetermined time interval, exhaustion of the desired fraction of carbon source supplied, cessation of carbon source consumption, or cessation of fermentation product formation. In one embodiment, “end of fermentation” is defined as the point in time where harvesting of the bioproduct is started. As would be understood by a person skilled in the art, “end of fermentation” can refer to a point in time that is different depending on the scale and purpose of the fermentation process. For a large-scale production fermentation process, the “end of fermentation” is preferably the point at which harvesting of the bioproduct is started, i.e., after product formation has effectively stopped.

As used herein, “cell dry weight” refers to the concentration of dry cell mass present in a fermentation medium at the time of measurement, as measured in a fermentation sample. Cell dry weight is commonly expressed in units of grams/liter (g/L).

As used herein, “cell dry weight at inoculation” refers to the concentration of dry cell mass present in a fermentation medium immediately following inoculation, as measured in a fermentation sample. For fed-batch fermentations, the initial cell dry weight is calculated based on the final volume of fermentation medium. Measurement of dry cell weight is a method known to those skilled in the art. Cell dry weight at inoculation is commonly expressed in units of g/L.

As used herein, “cell dry weight at end of fermentation” refers to the concentration of dry cell mass present in a fermentation medium at the end of fermentation, as measured in a fermentation sample. Cell dry weight at end of fermentation is commonly expressed in units of g/L.

As used herein, “final titer” refers to the concentration of a substance in the fermentation broth at the end of fermentation. The final titer is commonly expressed in units of g/L.

As used herein, “initial titer” refers to the concentration of a substance present at inoculation. The initial titer is commonly expressed in units of g/L.

As used herein, “batch time” refers to the amount of time that has elapsed between the inoculation and the end of fermentation. The batch time is commonly expressed in units of hours (h).

As used herein, “sugar consumption rate” for a batch process refers to the difference between the initial titer of a sugar present in the fermentation broth and the final titer of the same sugar (initial titer minus final titer) divided by the batch time. The sugar consumption rate is commonly expressed in units of grams per liter-hour (g L⁻¹ h⁻¹, which can also be abbreviated as (g/(L*h))). When applied to a continuous or semi-continuous process, the “sugar consumption rate” is determined using methods known in the art.

As used herein, the “specific sugar consumption rate” for a batch process refers to the sugar consumption rate divided by the cell dry weight at the end of fermentation. The specific sugar consumption rate is commonly expressed in units of (g sugar) (g cells)⁻¹ h⁻¹. When applied to a continuous or semi-continuous process, the “specific sugar consumption rate” is determined using methods known in the art.

The sugar consumption rate and specific sugar consumption rate may be applied to specific sugars such as, for instance, glucose or sucrose. In these cases, one may refer to a glucose consumption rate, specific glucose consumption rate, sucrose consumption rate, or specific sucrose consumption rate.

As used herein, “fermentation production rate” for a batch process refers to the final titer minus initial titer of fermentation product (final titer minus initial titer) divided by the batch time. The production rate is commonly expressed in units of grams per liter-hour (g L⁻¹ h⁻¹). When applied to a continuous or semi-continuous process, the “fermentation production rate” is determined using methods known in the art.

As used herein, the “specific production rate” refers to the fermentation production rate divided by the cell dry weight at the end of fermentation. The specific production rate is commonly expressed in units of (g product) (g cells)⁻¹ h⁻¹. When applied to a continuous or semi-continuous process, the “specific production rate” is determined using methods known in the art.

As used herein, “product yield” of a fermentation product refers to a ratio of two quantities: a) mass of product (e.g., succinate) produced in the course of the fermentation (numerator) b) the mass of carbon source added to the fermentation (denominator). The product yield as a percentage is commonly expressed in units of gram per gram (g/g) times 100. Particular note should be taken that product yield is calculated as a ratio of masses. The mass of fermentation product produced should account for the mass of fermentation product present in the fermentation medium at the end of the batch, as well as the mass of any fermentation product harvested during the course of the batch, less the mass of fermentation product present at the start of batch, and further less the mass of any fermentation product added during the course of the batch. The mass of carbon source added to the batch should include the mass of all carbon source(s) present in the fermenter at the start of the batch in addition to the mass of any carbon source(s) added during the course of the batch.

As used herein, “oxygen uptake rate” (“OUR”) refers to the volumetric rate at which oxygen is consumed during a fermentation. Inlet and outlet oxygen concentrations can be measured with exhaust gas analysis, for instance by mass spectrometers. OUR can be calculated by one of ordinary skill in the relevant arts using the Direct Method described in Bioreaction Engineering Principles 2nd Edition, 2003, Kluwer Academic/Plenum Publishers, p. 449, equation 1. It is commonly measured in units of (mmol o2) L⁻¹ h⁻¹.

As used herein, “specific oxygen uptake rate” refers to the specific rate at which oxygen is consumed during a fermentation. It is calculated as the ratio of the OUR to the measured cell dry weight. It is commonly measured in units of mmol o2 (g cell dry weight)⁻¹ h⁻¹.

As used herein, the term “microaerobic” refers to fermentation aeration conditions that are intermediate between fully aerobic and anaerobic conditions. Under microaerobic conditions, oxygen is supplied to the fermentation. Further, the oxygen is supplied at a rate such that the dissolved oxygen concentration is predominantly maintained below 5% of the saturation concentration of oxygen in the fermentation medium under air at atmospheric pressure. Under microaerobic conditions, the oxygen uptake rate is typically between 0.1 (mmol 0₂) L⁻¹ h⁻¹ and 40 (mmol 0₂) L⁻¹ h⁻¹

Yeast Characteristics Definitions

As used herein, the term “Crabtree-negative” refers to a yeast cell having a Crabtree-negative phenotype, i.e., any yeast cell that does not exhibit the Crabtree effect. In one embodiment, the host cell of the present invention is a Crabtree-negative yeast. The Crabtree effect concerns the inhibition of synthesis of respiratory enzymes. The Crabtree effect is defined as the occurrence of fermentative metabolism under aerobic conditions as a result of the inhibition of oxygen consumption by a microorganism when cultured at high specific growth rates (long-term effect) or in the presence of high concentrations of glucose (short-term effect). Organisms with the Crabtree negative phenotype do not exhibit this effect, and are thus able to consume oxygen even in the presence of high concentrations of glucose or at high growth rates. Whether an organism is Crabtree positive or Crabtree negative can be determined by comparing the ratio of fermented glucose to respired glucose during cultivation under aerobic conditions, with a ratio of greater than 1 indicative of a Crabtree positive organism (e.g., see De Deken, R. H. (1965) J. gen. Microbiol., 44:149-156).

As used herein, “sugar capacity” refers to the rate at which a yeast consumes a sugar as measured according to the method titled “strain capacity evaluation” as described below. The sugar capacity refers to the difference between the initial titer of a sugar present in the fermentation broth and the titer of the same sugar at the end of the evaluation (initial titer minus end titer) divided by the batch time, further divided by the cell dry weight at the end of the evaluation. The sugar capacity is commonly expressed in units of (g sugar) (g cells)⁻¹ h⁻¹. This assay can be used to measure the sugar capacity for a number of sugars such as glucose or sucrose, resulting in, for example, a measurement of “glucose capacity” or “sucrose capacity.”

${sugar}\mspace{14mu} {capacity}{= \frac{{\lbrack{sugar}\rbrack {initial}} - {\lbrack{sugar}\rbrack_{end}\mspace{14mu} {of}\mspace{14mu} {evaluation}}}{\left( {{batch}\mspace{14mu} {time}} \right) \times \left( {{cell}\mspace{14mu} {dry}\mspace{14mu} {weight}\mspace{14mu} {at}\mspace{14mu} {end}\mspace{14mu} {of}\mspace{14mu} {evaluation}} \right)}}$

For example, in an evaluation that lasts 45 hours, with 140.0 g/L glucose present at inoculation, 1.0 g/L glucose present at the end of the evaluation, and 6.0 g/L cell dry weight of yeast present at the end of fermentation, the calculated glucose capacity is 0.51 g glucose g⁻¹ cells h⁻.

As used herein, “product capacity” refers to the rate at which a yeast produces a fermentation product as measured according the method titled “strain capacity evaluation” as described below. The product capacity refers to the difference between the initial titer of a product present in the fermentation broth and the titer of the same product at the end of the evaluation (initial titer minus end titer) divided by the batch time, further divided by the cell dry weight at the end of the evaluation. The product capacity is commonly expressed in units of (g product) (g cells)⁻¹ h⁻¹. This assay can be used to measure the product capacity for a number of products such as lactate or succinate, resulting in a measurement of, for example, “lactate capacity” or “succinate capacity.”

${{product}\mspace{14mu} {capcity}} = \frac{{\left\lbrack {p^{r}{\,^{\; o}{duct}}} \right\rbrack_{end}\mspace{14mu} {of}\mspace{14mu} {evaluation}} - {\left\lbrack {p^{r}{\,^{\; o}{duct}}} \right\rbrack \; i_{n}{itial}}}{\left( {{batch}\mspace{14mu} {time}} \right) \times \left( {{cell}\mspace{14mu} {dry}\mspace{14mu} {weight}\mspace{14mu} {at}\mspace{14mu} {end}\mspace{14mu} {of}\mspace{14mu} {evaluation}} \right)}$

For example, in an evaluation that lasts 45 hours, with 0.0 g/L succinate present at inoculation, 100.0 g/L succinate present at the end of the evaluation, and 6.0 g/L cell dry weight of yeast present at the end of fermentation, the calculated succinate capacity is 0.37 g glucose g⁻¹ cells h⁻¹.

As used herein, “ratio of invertase activity to glucose capacity” refers to the ratio of invertase activity of a yeast strain, as measured according to the “invertase activity evaluation” method described below, to the observed glucose capacity of the same strain, as measured according to the “strain capacity evaluation” method described below. The units of this parameter are (g glucose released from sucrose hydrolysis/(g cell dry weight*hour))/(g glucose consumed/(g cell dry weight*hour)).

In certain embodiments, the genetically modified yeast cells provided herein further comprise a deletion or disruption of one or more native genes. As used herein, the phrase “deletion or disruption” with regard to a native gene means that either the entire coding region of the gene is eliminated (deletion) or the coding region of the gene, its promoter, and/or its terminator region is modified (such as by deletion, insertion, or mutation) such that the gene no longer produces an active enzyme, produces a severely reduced quantity (at least 75% reduction, preferably at least 90% reduction) of an active enzyme, or produces an enzyme with severely reduced (at least 75% reduced, preferably at least 90% reduced) activity.

In certain embodiments, deletion or disruption of one or more native genes results in a deletion or disruption of one or more native metabolic pathways. The phrase “deletion or disruption” with regard to a metabolic pathway means that the pathway is either inoperative or else exhibits activity that is reduced by at least 75%, at least 85%, or at least 95% relative to the native pathway. In certain embodiments, deletion or disruption of a native metabolic pathway is accomplished by incorporating one or more genetic modifications that result in decreased expression of one or more native genes that reduce ethanol production.

In some embodiments, deletion or disruption of native genes can be accomplished by forced evolution, mutagenesis, or genetic engineering methods, followed by appropriate selection or screening to identify the desired mutants. In some embodiments, deletion or disruption of a native host cell gene can be coupled to the incorporation of one or more exogenous genes into the host cell, i.e., the exogenous genes can be incorporated using a gene expression integration construct that is also a deletion construct. In some embodiments, deletion or disruption can be accomplished using a deletion construct that does not contain an exogenous gene or by other methods known in the art.

In some embodiments, the modified yeast cells described herein have a deletion or disruption of one or more native genes encoding an enzyme involved in ethanol fermentation or consumption, including for example pyruvate decarboxylase (PDC, catalyzes the conversion of pyruvate to acetaldehyde and carbon dioxide). Such modifications decrease the ability of the yeast cell to produce ethanol, thereby maximizing fermentation product production. In some embodiments where the modified yeast cell is I. orientalis, the cells comprise a deletion or disruption of a PDC gene encoding the amino acid sequence of SEQ ID NO: 14 and/or a gene encoding an amino acid sequence with at least 50%, at least 60%, at least 70%, at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, or at least 99% sequence identity to the amino acid sequence of SEQ ID NO: 14.

As used herein, the terms “PDC-negative” or “PDC-” refer to a yeast which has a deletion or disruption of the pyruvate decarboxylase (PDC) gene. As would be understood by a person skilled in the art, deletion or disruption of the PDC gene will eliminate or reduce expression of PDC enzyme, which is an enzyme necessary for the production of ethanol via fermentation. In one embodiment, the pyruvate decarboxylase activity of the yeast is less than 0.05 U/milligram of total protein when using the methods previously described by Michele M. Bianchi, Lorenza Tizzani, Monika Destruelle, Laura Frontal and Micheline Wesolows ki-Louvel, The ‘petite-negative’ yeast Kluyveromyces lactis has a single gene expressing pyruvate decarboxylase activity. (1996) Molecular Microbiology, 19 (1): 27-36. Biomass used for the assay is grown in YP media with 2% glucose. The activity unit (U) is defined as the amount of activity required for the conversion of 1 micromole of substrate (in this example, NADH to NAD+) per minute.

The term “exogenous” as used herein with regard to genetic components means that the genetic component is present in a modified version of a microorganism, but is not present in the genome of a native form of the particular microorganism cell. In some embodiments, the exogenous genetic component can be a modified form of a component that was native to the cell, it can be derived from another organism, it can be a modified form of a component derived from another organism, or it can be a synthetically-derived component. For example, the K. lactis invertase gene is exogenous when introduced into I. orientalis.

Inspection of nucleic acid or amino acid sequences for two nucleic acids or two polypeptides will reveal sequence identity and similarities between the compared sequences. Sequence alignment and generation of sequence identity include global alignments and local alignments which are carried out using computational approaches. An alignment can be performed using BLAST (National Center for Biological Information (NCBI) Basic Local Alignment Search Tool) version 2.2.31 software with default parameters. Amino acid % sequence identity between amino acid sequences can be determined using standard protein BLAST with the following default parameters: Max target sequences: 100; Short queries: Automatically adjust parameters for short input sequences; Expect threshold: 10; Word size: 6; Max matches in a query range: 0; Matrix: BLOSUM62; Gap Costs: (Existence: 11, Extension: 1); Compositional adjustments: Conditional compositional score matrix adjustment; Filter: none selected; Mask: none selected. Nucleic acid % sequence identity between nucleic acid sequences can be determined using standard nucleotide BLAST with the following default parameters: Max target sequences: 100; Short queries: Automatically adjust parameters for short input sequences; Expect threshold: 10; Word size: 28; Max matches in a query range: 0; Match/Mismatch Scores: 1, −2; Gap costs: Linear; Filter: Low complexity regions; Mask: Mask for lookup table only. A sequence having an identity score of XX % (for example, 80%) with regard to a reference sequence using the NCBI BLAST version 2.2.31 algorithm with default parameters is considered to be at least XX % identical or, equivalently, have XX % sequence identity to the reference sequence.

Throughout this disclosure, various aspects of the invention may be presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible subranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 7 should be considered to have specifically disclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 6, from 2 to 5, from 3 to 5, etc., as well as individual numbers within that range, for example, 1, 2, 3, 3.6, 4, 5, 5.8, 6, 7, and any whole and partial increments in between. This applies regardless of the breadth of the range.

DESCRIPTION

Described herein are genetically modified yeast strains useful for manufacturing a fermentation product and fermentation processes using these yeasts. The yeast strains are modified to include a functional exogenous invertase gene. Accordingly, in one embodiment, the present invention relates to a yeast strain useful for fermentation processes having sucrose as a substrate. The yeast strain is preferably PDC-negative, and therefore can be useful for manufacturing fermentation products other than ethanol, for example succinic acid. In one embodiment, the yeast is Crabtree negative.

As contemplated herein, sucrose-based fermentation processes would preferably use a yeast expressing the invertase enzyme. However, invertase expression is not native to many yeasts that are desirable for industrial fermentation processes. Feng et al., describe the relationship between the fermentation activity of Saccharomyces cerevisiae in high-sugar dough and sucrase activity (Modern Food Sci. and Tech., 2014, 30:131-135). However, S. cerevisiae is primarily used for the production of ethanol, i.e., it has pyruvate decarboxylase (PDC) activity, and it is less desirable for use in manufacturing many other types of industrial chemicals. As would be understood by a person of ordinary skill in the art, deletion or disruption of the PDC gene in S. cerevisiae is highly problematic. This deletion in S. cerevisiae results in the loss of the ability to grow on glucose, as well as causing an autotrophy for c2 compounds (Flikweert et al., Growth requirements of pyruvate-decarboxylase-negative Saccharomyces cerevisiae, FEMS Microbiol Lett 1999; 174(1):73-9).

Genetically Engineered Yeast

The genetically modified yeast of the present invention is made by performing one or more genetic modifications to a host yeast cell. In some embodiments, the host yeast cell lacks a native invertase. In some embodiments, the host yeast cell does not include a nucleic acid encoding a polypeptide with a sequence that has greater than 70% identity with SEQ ID NO: 6, SEQ ID NO: 15, SEQ ID NO: 16, or SEQ ID NO: 17. In some embodiments, the host yeast cell cannot grow on sucrose as a sole carbon source. In some embodiments, the host yeast cell has a maximum specific growth rate on (YNB+20 g/L glucose) media that exceeds 0.15 h⁻¹ and a maximum specific growth rate on (YNB+20 g/L sucrose) media that is less than 0.05 h⁻¹. In some embodiments, the host yeast is a Crabtree-negative yeast.

In some embodiments, the genetically modified yeast cells described herein belong to the genus Issatchenkia, and in some such embodiments the yeast cells are I. orientalis. When first characterized, the species I. orientalis was assigned the name Pichia kudriavzevii. I. orientalis yeasts have also been described in the art as C. krusei. Numerous additional synonyms for the species I. orientalis have been described (see Kurtzman and Fell, The Yeasts, a Taxonomic Study, Section 35, Issatchenkia Kudryavtsev, pp. 222-223 (1998), which is hereby incorporated by reference).

The I. orientalis/P. fermentans clade is the most terminal clade that contains at least the species I. orientalis, Pichia galeiformis, Pichia sp. YB-4149 (NRRL designation), Candida ethanolica, Pichia deserticola, P. membranifaciens, and P. fermentans. Members of the I. orientalis/P. fermentans clade are identified by analysis of the variable D1/D2 domain of the 26S ribosomal DNA of yeast species, using the method described by Kurtzman and Robnett in “Identification and Phylogeny of Ascomycetous Yeasts from Analysis of Nuclear Large Subunit (26S) Ribosomal DNA Partial Sequences,” Antonie van Leeuwenhoek 73:331-371, 1998, which is hereby incorporated by reference (see especially p. 349). Analysis of the variable D1/D2 domain of the 26S ribosomal DNA from hundreds of ascomycetes has shown that the I. orientalis/P. fermentans clade contains very closely related species. Members of the I. orientalis/P. fermentans clade exhibit greater similarity in the variable D1/D2 domain of the 26S ribosomal DNA to other members of the clade than to yeast species outside of the clade. Therefore, other members of the I. orientalis/P. fermentans clade can be identified by comparison of the D1/D2 domains of their respective ribosomal DNA, and comparing to that of other members of the clade and closely related species outside of the clade, using Kurtzman and Robnett's methods.

As described herein, the present invention relates to genetically modified yeasts of the I. orientalis/P. fermentans clade, preferably I. orientalis. However, the present invention is not limited to using any specific yeast such as I. orientalis, and the host yeast cell can be any suitable yeast strain, as would be understood by a person skilled in the art. To genetically modify the yeast cell, a suitable locus is selected for gene integration. One of ordinary skill in the art would know how to select suitable loci in a yeast genome for gene integration. An example of a suitable locus for integration of exogenous genes in I. orientalis includes, but is not limited to, locus A, which is flanked by SEQ ID NO: 1 and SEQ ID NO: 2. Further, one of ordinary skill in the art would recognize how to use sequences to design PCR primers to verify correct gene integration at the chosen locus.

As contemplated herein, the genetically modified or engineered yeast of the present invention includes a functional exogenous invertase expression gene and has a deletion or disruption of the PDC gene. In one embodiment, the genetically modified yeast can include one or more additional exogenous integrated genes other than the integrated functional invertase expression gene. In one embodiment, the genetically modified yeast can include more than one functional invertase expression gene. In another embodiment, the genetically modified yeast can include a functional sucrase gene instead of, or in addition to, the invertase gene. For the purposes of this disclosure, an integrated gene does not include a gene maintained on a plasmid.

Exemplary invertase expression genes suitable for gene integration in a yeast strain include, but are not limited to: an invertase gene from K. lactis (KIIN V); S. cerevisiae (ScSUC2); Schizosaccharomyces pombe (invl); and Aspergillus niger (invA) also identified as SEQ ID NO: 6; SEQ ID NO: 15; SEQ ID NO: 16; and SEQ ID NO: 17, respectively.

The genetically modified yeast of the present invention can also include exogenous or artificial promoters for the functional exogenous invertase expression gene or any other gene integrated into the yeast. One skilled in the art would know how to select and integrate suitable promoters into the host yeast cell. Examples of suitable promoters include, but are not limited to the promoters for the following I. orientalis genes: Pyruvate Decarboxylase (PDC), Glyceraldehyde-3-phosphate dehydrogenase (TDH3), Translational elongation factor (TEF), Transaldolase (TAL), RPL16B, 3-phosphoglycerate kinase (PGK), and Enolase (ENO).

In some embodiments, the integrated functional exogenous invertase expression may be associated with invertase activity which, once integrated into the host yeast cell, can be significantly greater than the desirable or optimal invertase activity. Greater than desired invertase activity can result in a less than optimal fermentation process. Greater than desired invertase activity can be problematic for a host cell and result in a reduction in the sugar consumption rate of the cell. While not wishing to be bound by theory, this reduction in sugar consumption rate can be due to the metabolic burden associated with producing large quantities of invertase protein, or can be due to other reasons that are not well understood.

Accordingly, the present invention also relates to the adjustment of invertase expression associated with the genetically modified yeast. Invertase expression in the genetically modified yeast can be optimized through one or more techniques known in the art. For example, in one embodiment, the amino acid sequence of invertase can be modified to reduce activity. In another embodiment, promoters associated with lower expression of invertase can be identified and integrated into the host yeast. However, the methods and compositions for optimizing invertase expression are not limited to those described herein, and can include any methods or compositions for adjusting or optimizing the invertase expression, as would be understood by a person skilled in the art.

In some embodiments, the yeast can be engineered for improved acetate consumption. Acetate consumption can be improved by overexpression of a gene encoding for an aldehyde dehydrogenase, or an acetyl-CoA synthase. In some embodiments, acetate consumption can be further improved by providing the cell with a greater pool of reducing equivalents to assist in the oxido-reduction of acetate to ethanol. One example of a genetic modification that can increase the pool of reducing equivalents is the deletion or disruption of a gene encoding a glycerol-3-phosphate dehydrogenase (GPD).

In some embodiments, the yeast can include heterologous expression of a transporter that can increase hexose uptake. An example of a transporter than can increase hexose uptake is Hxtl transporter of S. cerevisiae. One skilled in the art would recognize that yeasts are known to have other transporters capable of hexose uptake.

In some embodiments, the genetically engineered yeast of the present invention is capable of manufacturing a fermentation product other than ethanol. In some embodiments, the yeast is capable of producing a fermentation product at a production rate of at least 1.0 grams per liter-hour (g L⁻¹ h⁻¹), at least 1.5 g L⁻¹ h⁻¹, or at least 2.0 g L⁻¹ h⁻¹. In some embodiments, the yeast is capable of producing a fermentation product at a pathway fermentation yield of at least 55 percent, at least 65 percent, at least 70 percent, or at least 75 percent. In some embodiments, the yeast is capable of producing a fermentation product at a final titer of at least 30 g/liter, at least 80 g/liter, or at least 100 g/liter.

As contemplated herein, the genetically engineered yeast is capable of producing a fermentation product using sucrose as a fermentation substrate. The ratio of invertase activity to the rate of glucose consumption via fermentation can be optimized to maximize the manufacture of fermentation product. In some embodiments, the yeast has a ratio of invertase activity to glucose capacity of less than 95, less than 90, less than 80, less than 70, less than 60, less than 50, less than 40, less than 35, less than 30, less than 25 or less than 20. In some embodiments, the yeast has a ratio of invertase activity to glucose capacity of at least 0.95 or at least 10. In some embodiments, the yeast has a ratio of invertase activity to glucose capacity of at least 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, or 18. In some embodiments, the yeast has a ratio of invertase activity to glucose capacity in the range of 0.5 to 95, 0.5 to 30, 0.5 to 25, 0.5 to 20, or 1 to 20.

In some embodiments, the invertase activity of the yeast is at least 1, 2, 2.5, 3, 4, 5, 6, 7, 8, or 9 g glucose released/ (g CDW*h). In some embodiments, the invertase activity of the yeast is less than 10, 15, 20, 30, 40, or 50 g glucose released/(g CDW*h). In some embodiments, the invertase activity of the yeast is in the range of 1 to 50, 2.5 to 50, 2.5 to 25, 3 to 30, 5 to 30, 3 to 20, or 5 to 20.

The yeast can also be capable of producing a fermentation product using other fermentation substrates in addition to sucrose. In one embodiment, the yeast is capable of using a fermentation substrate that includes sucrose and glucose. In another embodiment, the yeast is capable of using a fermentation substrate that includes sucrose and xylose. In yet another embodiment, the yeast is capable of using a fermentation substrate that includes sucrose, glucose, and xylose. In some embodiments, the yeast is capable of using a fermentation substrate that includes hydrozylates, for example hydrozylates of starch or lignocellulosic hydrozylates. In some embodiments, the yeast is capable of using a fermentation substrate that includes any mixture or combination of sucrose, glucose, fructose, xylose, hydrozylates of starch, or lignocellulosic hydrozylates. As would be understood by a person skilled in the art, the yeast can be used with a fermentation substrate that does not include sucrose.

In one embodiment, the yeast of the present invention can include one or more inducible promoters. For example, the yeast may include a promoter capable of turning off invertase expression after most or all of the sucrose in the fermentation substrate has been hydrolyzed. As a further example, the yeast may contain a promoter that is capable of down regulating after the dissolved oxygen is reduced below a threshold.

Fermentation Processes

The present invention also relates to processes for manufacturing a fermentation product. The fermentation processes includes the step of fermenting a substrate using the genetically engineered yeasts described herein. The fermentation process can also include other steps, as would be understood by a person skilled in the art. Non-limiting examples of additional process steps include maintaining the temperature of the fermentation broth within a predetermined range, adjusting the pH during fermentation, and isolating the fermentation product from the fermentation broth. In some embodiments, the fermentation process is a microaerobic process.

The fermentation processes of the present invention can be run using sucrose as a substrate, as a result of using genetically engineered yeasts having a functional invertase gene. The substrate of the fermentation process can also include other components in addition to sucrose. In one embodiment, the fermentation process substrate can also include glucose, xylose, fructose, hydrozylates of starch, lignocellulosic hydrozylates, or any combination thereof. As contemplated herein, the sucrose component of the substrate will be hydrolyzed into glucose and fructose via the activity of invertase and/or sucrase. Accordingly, in some embodiments, the fermentation substrate may not contain any sucrose because all of the sucrose may be hydrolyzed at some point during the process.

The fermentation process can be run under various conditions. In one embodiment, the fermentation temperature, i.e., the temperature of fermentation broth during processing, is ambient temperature. In some embodiments, the fermentation temperature is maintained within a predetermined range. For example, the fermentation temperature can be maintained in the range of 25 to 45° C., 20 to 40° C., or 33 to 38° C. However, the fermentation temperature is not limited to any specific range recited herein.

The fermentation process can be run within certain oxygen uptake rate (OUR) ranges. In some embodiments, the volumetric OUR of the fermentation process can be in the range of 0.5 to 40, 1 to 30, 3 to 20, or 5 to 16 mmol 0 2/(L-h). In some embodiments, the specific OUR can be in the range of 0.2 to 13, 0.3 to 10, 1 to 7, or 2 to 6 mmol O{circumflex over ( )}g cell dry weight-h). However, the volumetric or specific OURs of the fermentation process are not limited to any specific rates or ranges recited herein.

The fermentation process can be run at various cell concentrations. In some embodiments, the cell dry weight at the end of fermentation can be 1 to 20, 1 to 10, 2 to 8, or 2.5 to 6 g cell dry weight/L. Further, the pitch density or pitching rate of the fermentation process can vary. In some embodiments, the pitch density can be 0.05 to 5, 0.05 to 4, or 0.05 to 2 g cell dry weight/L.

In addition, the fermentation process can be associated with various characteristics, such as, but not limited to, fermentation production rate, pathway fermentation yield, final titer, and the ratio of invertase activity to glucose consumption rate. In some embodiments, these characteristics can be affected based on the selection of the yeast and/or genetic modification of the yeast used in the fermentation process. In some embodiments, these characteristics can be affected by adjusting the fermentation process conditions. In some embodiments, these characteristics can be adjusted via a combination of yeast selection or modification and the selection of fermentation process conditions.

In some embodiments, the fermentation production rate of the process is at least 1.0, at least 1.5, or at least 2.0 g L⁻¹ h⁻¹. In some embodiments, the pathway fermentation yield of the process is at least 55 percent, at least 65 percent, at least 70 percent, or at least 75 percent. In some embodiments, the final titer of the process is at least 30, at least 80, or at least 100 g/liter. In some embodiments, the process has a ratio of invertase activity to glucose consumption rate of less than 95, less than 90, less than 80, less than 70, less than 60, less than 50, less than 40, less than 35, less than 30, less than 25 or less than 20. In some embodiments, the process has a ratio of invertase activity to glucose consumption rate of at least 0.95 or at least 10. In some embodiments, the process has a ratio of invertase activity to glucose consumption rate of at least 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 15, or 18. In some embodiments, the process has a ratio of invertase activity to glucose consumption rate in the range of 0.5 to 95, 0.5 to 30, 0.5 to 25, 0.5 to 20, or 1 to 20.

In some embodiments, the invertase activity of the process is at least 1, 2, 2.5, 3, 4, 5, 6, 7, 8, or 9 g glucose released/(g CDW*h). In some embodiments, the invertase activity of the process is less than 10, 15, 20, 30, 40, or 50 g glucose released/(g CDW*h). In some embodiments, the invertase activity of the process is in the range of 1 to 50, 2.5 to 50, 2.5 to 25, 3 to 30, 5 to 30, 3 to 20, or 5 to 20.

In some embodiments, the fermentation process can include sucrose as a substrate for only a portion of the process. For example, in one embodiment, the fermentation process can include the step of generating a yeast seed using sucrose as substrate, then running the full production batch with a hydrolysate, a hydrolysate supplemented with sucrose, or other substrate instead of sucrose. In one such embodiment, the fermentation process can be run as a sucrose-fed batch. Further, the fermentation process can be a batch process, continuous process, or semi-continuous process, as would be understood by a person skilled in the art.

Fermentation Products

The genetically engineered yeast of the present invention and the fermentation processes using the genetically engineered yeast can be used to manufacture a variety of compounds. Exemplary fermentation products that can be manufactured using the genetically engineered yeast include, but are not limited to: amino acids, organic acids, hydroxyl-organic acids, alcohols such as butanol, polyols, fatty acids, fatty acids such as methyl esters, monoacyl glycerides, diacyl glycerides, triacyl glycerides, and mixtures thereof. Exemplary organic acids or amino acids include lactic acid, citric acid, malonic acid, hydroxy butyric acid, adipic acid, lysine, keto-glutaric acid, glutaric acid, 3-hydroxy-proprionic acid, succinic acid, malic acid, fumaric acid, itaconic acid, muconic acid, methacrylic acid, and acetic acid and derivatives thereof and salts thereof. It is contemplated herein that isolation of the desired fermentation product produced from the fermentation process can be achieved via techniques well known to those skilled in the relevant art.

EXPERIMENTAL EXAMPLES

The invention is further described in detail by reference to the following experimental examples. These examples are provided for purposes of illustration only, and are not intended to be limiting unless otherwise specified. Thus, the invention should in no way be construed as being limited to the following examples, but rather should be construed to encompass any and all variations which become evident as a result of the teaching provided herein.

Evaluation Protocols Strain Capacity Evaluation

The following protocol is used for evaluating the sugar capacity or product capacity of a yeast strain, as defined herein. Fermenters are inoculated with biomass grown in defined medium (adapted from Verduyn, et. al, 1992, Yeast 8, 501-517, see Tables 1, 3, and 4). Seeds are run in 250 mL baffled flasks (50 mL working volume) at 250 rpm and 30° C. The contents of the flasks are harvested at approximately 16-24 hours incubation time. The cell density of the shake flask is measured and a volume of the shake flask broth is selected and inoculated into the fermenter such that the cell dry weight at inoculation is 0.1 g/L. Fermenter initial working volume is 1.5 L. Fermenter media is used as listed in Tables 2, 3, and 4. Sugar is provided by the addition of 140 g/l at the start of the batch (straight batch). pH is started at the ambient pH of the media (4-6) and is controlled at 3.0 with a combination of 28% NH₄OH and 30% Ca(OH)₂. 3.8 g per 1.5 L media 28% NH4OH is used as initial pH control. Once this is exhausted, pH control is switched to Ca(OH)2 for the remainder of the batch. The fermenter systems are sparged at 0.24 slpm with a blend of pure CO2 and air to target 21-23% CO2 in the inlet gas stream. The fermentation is operated such that after the cells achieve a sufficient density, oxygen limitation is achieved and subsequently maintained throughout the rest of the fermentation (e.g., dissolved oxygen less than about 10%). Agitation rate is selected to achieve a peak oxygen uptake rate (OUR) in the fermentation of 21-22 mmol/L-h. The fermentation proceeds until the end of the evaluation which occurs when the sugar is reduced below 2 g/L or until the cessation of product formation, whichever occurs first. Samples are taken immediately after inoculation and at the end of the evaluation. These samples are used for cell dry weight determination as well as HPLC analysis for determination of sugar and product concentrations.

Invertase Activity Evaluation

The capability of a cell to convert sucrose to glucose and fructose is evaluated by the following protocol. The strains are taken from a fresh YPD plate and used to inoculate 50 mL of YPD liquid media. The culture is allowed to grow at 30° C./250 rpm overnight (16 hours). Fresh cultures are inoculated to an OD6oo=1.0 in 50 mL of YPD liquid media and allowed to grow at 30° C./250 rpm for 3 hours. The cells are harvested by centrifugation at 3,500 rpm for 4 minutes. The pellets are washed with 25 mL of water and centrifuged at 3,500 rpm for 4 minutes; this step is repeated 2 times. Washed cells are resuspended in 5 mL of water. 10 μL of cell suspension is incubated with 40 μL water, 250 μL of 0.2 M sodium acetate, pH 4.9 and 125

of 0.5 M sucrose for 10 min at 37° C. Samples are filtered through a 0.22 μm filter. The glucose released is immediately measured on a YSI2950 (Xylem Inc.). The activity is expressed as grams of glucose released per gram of cell dry weight/hour. Assays are carried out in duplicate.

This assay is adapted from Silveira, M. C. F., Carvajal, E., Bon, E. P. S., Assay for in vivo yeast invertase activity using NaF (1996) Analytical Biochemistry, 238 (1), pp. 26-28, and Georis, I., Cassart, J.-P., Breunig, K. D., Vandenhaute, Glucose repression of the Kluyveromyces lactis invertase gene KIINV1 does not require Miglp (1999), Molecular and General Genetics 261(4-5):862-70.

Example 1: Genetically Modified Yeast Strains Strain 1-1

Strain P-8b described by Rush et al. (Int'l. App. No. PCT/US2013/052069) is an evolved Issachenkia orientalis host strain in which both alleles of the URA3, PDC and GPD genes are deleted followed by the addition of diploid alleles of the following genes under control of heterologous promoters: I. orientalis PYC1, Schizosaccharomyces pombe MAE, Leshmania mexicana FRD, Rhizopus delemar MDH, and I. orientalis FUM (SEQ ID NO: 4). Strain 1-1 is created using the methods to create strain P-8b with the following change: 1) In Strain 1-1, the L. mexicana FRD gene of P-8b is replaced with the variant of the L. mexicana FRD gene of SEQ ID NO: 3.

Strain 1-1a

Strain 1-1 is grown for several rounds on 5-fluoroorotic acid (FOA) plates to identify a strain in which the URA3 marker has looped out. Resulting isolates are streaked for single colony isolation on YPD plates. A single colony is selected. Loss of the URA3 marker is verified by PCR. A PCR verified isolate is designated Strain 1-1a.

Strain 1-2

Strain 1-1a is transformed with SEQ ID NO: 5. SEQ ID NO: 5 contains: i) an expression cassette for the selectable marker gene URA from I. orientalis (IoURA) including a repeated portion of the URA promoter; ii) an expression cassette for an invertase from K. lactis (KIINV), encoding the amino acid sequence SEQ ID NO: 6 expressed by the PDC promoter SEQ ID NO: 7; and iii) flanking DNA for targeted chromosomal integration into integration locus A. Transformants are selected on ScD-Uracil plates. Resulting transformants are streaked for single colony isolation on ScD-Uracil plates. A single colony is selected. Correct integration of SEQ ID NO: 5 into the selected colony is verified by PCR. A PCR verified isolate is designated Strain 1-2.

Strain 1-3

Strain 1-1a is transformed with SEQ ID NO: 8. SEQ ID NO: 8 contains: i) an expression cassette for the selectable marker gene URA from I. orientalis (IoURA) including a repeated portion of the URA promoter; ii) an expression cassette for an invertase from K. lactis (KIINV), encoding the amino acid sequence SEQ ID NO: 6 expressed by the TAL promoter SEQ ID NO: 9; and iii) flanking DNA for targeted chromosomal integration into integration locus A. Transformants are selected on ScD-Uracil plates. Resulting transformants are streaked for single colony isolation on ScD-Uracil plates. A single colony is selected. Correct integration of SEQ ID NO: 8 into the selected colony is verified by PCR. A PCR verified isolate is designated Strain 1-3.

Strain 1-4

Strain 1-1a is transformed with SEQ ID NO: 10. SEQ ID NO: 10 contains: i) an expression cassette for the selectable marker gene URA from I. orientalis (IoURA) including a repeated portion of the URA promoter; ii) an expression cassette for an invertase from K. lactis (KIINV), encoding the amino acid sequence SEQ ID NO: 6 expressed by the RPL16B promoter SEQ ID NO: 11; and iii) flanking DNA for targeted chromosomal integration into integration locus A. Transformants are selected on ScD-Uracil plates. Resulting transformants are streaked for single colony isolation on ScD-Uracil plates. A single colony is selected. Correct integration of SEQ ID NO: 10 into the selected colony is verified by PCR. A PCR verified isolate is designated Strain 1-4.

Strain 1-5

Strain P-8b as described in the section titled “Strain 1-1” above is co-transformed with the integration fragments 6-1 and 6-2 listed in the second column of Table 3 in Rush et al. (Int'l. App. No. PCT/US20 13/052069). Integration fragments 6-1 and 6-2 target the E. coli transhydrogenase gene to the GPD locus. Successful integrants in each case are identified as blue colonies on selection plates with 5-bromo-4-chloro-3-indolyl-alpha-D-galactopyranoside and lacking uracil, and confirmed by PCR. A PCR verified isolate is designated Strain 1-5.

Strain 1-6

Strain 1-5 is transformed with the plasmid of SEQ ID NO: 12. SEQ ID NO: 12 contains: i) an expression cassette for the selectable marker gene invertase from S. cerevisiae (ScSUC2); and ii) an expression cassette for CRE recombinase gene (Cre) to recycle the selectable markers ScMEL5 & IoCYB2A. Transformants are selected on YNB plates containing 2% sucrose as sole carbon source and 32 μg/mL x-alpha-gal which provides colorimetric indication of the absence of the ScMEL5 marker gene. Resulting transformants are streaked for single colony isolation on YPD containing 32 μg/mL x-alpha-gal. A single white colony is selected. Loss of ScMEL5 and IoCYB2A from the selected white colony is verified by PCR. A PCR verified isolate is designated Strain 1-5a.

Strain 1-5a is grown for several rounds on 5-fluoroorotic acid (FOA) plates to identify a strain in which the URA3 marker has looped out. Resulting isolates are streaked for single colony isolation on YPD plates. A single colony is selected. Loss of the URA3 marker is verified by PCR. A PCR verified isolate is designated Strain 1-6.

Strain 1-7

Strain 1-6 is transformed with SEQ ID NO: 10. SEQ ID NO: 10 contains: i) an expression cassette for the selectable marker gene URA from I. orientalis (IoURA) including a repeated portion of the URA promoter; ii) an expression cassette for an invertase from K. lactis (KIINV), encoding the amino acid sequence SEQ ID NO: 6 expressed by the RPL16B promoter SEQ ID NO: 11; and iii) flanking DNA for targeted chromosomal integration into integration locus A. Transformants are selected on ScD-Uracil plates. Resulting transformants are streaked for single colony isolation on ScD-Uracil plates. A single colony is selected. Correct integration of SEQ ID NO: 10 into the selected colony is verified by PCR. A PCR verified isolate is designated Strain 1-7.

Strain 1-8

Strain 1-7 is transformed with SEQ ID NO: 13. SEQ ID NO: 13 contains: 1) an expression cassette for the selectable marker gene melibiase from S. cerevisiae (ScMEL5) flanked by LoxP sites; ii) an expression cassette for an invertase from K. lactis (KIINV), encoding the amino acid sequence SEQ ID NO: 6 expressed by the RPL16B promoter SEQ ID NO: 11; and iii) flanking DNA for targeted chromosomal integration into integration locus A. Transformants are selected on YNB plates containing 2% melibiose as sole carbon source and 32 μg/mL x-alpha-gal which provides colorimetric indication of the presence of the ScMEL5 marker gene. Resulting transformants are streaked for single colony isolation on YPD containing 32 μg/mL x-alpha-gal. A single blue colony is selected. Correct integration of SEQ ID NO: 13 into the selected blue colony is verified by PCR. A PCR verified isolate is designated Strain 1-8.

Example 2: Fermentation Using Genetically Modified Yeast Strains

This Example demonstrates the capability of the recombinant yeast strains having an exogenous invertase activity gene described above to convert sucrose to glucose and fructose, and subsequently and/or concurrently convert glucose to a fermentation product such as succinic acid.

Fermentation Conditions for Strains 1-1, 1-2, 1-3, and 1-4

The yeast strains 1-1, 1-2, 1-3, and 1-4, are run in fermenters to test succinic acid production. Fermenters are inoculated with biomass grown in defined medium (adapted from Verduyn, et. al, 1992, Yeast 8, 501-517, see Tables 1, 3, and 4). Seeds are run in 250 mL baffled flasks (50 mL working volume) at 250 rpm and 30° C. The contents of the flasks are harvested at approximately 16-24 hours incubation time with 10% v/v inoculum used to start fermenters. Fermenter initial working volume is 1.5 L. The cell dry weight at inoculation is found in Table 5. Fermenter media is outlined in Tables 2, 3, and 4. Glucose or sucrose is added to achieve a concentration of 140 g/L at the start of the batch (straight batch). pH is started at the ambient pH of the media (4-6) and is allowed to drop to pH 3.0, after which it is controlled at 3.0 for the remainder of the batch with a combination of 28% NH₄OH and 30% Ca(OH)2. 3.8 g per 1.5 L media 28% NH4OH is used as initial pH control. Once this is exhausted, pH control is switched to Ca(OH)2 for the remainder of the batch. The fermenter systems are sparged at 0.24 slpm with a blend of pure CO2 and air to target 21-23% CO2 in the inlet gas stream. These fermentations are operated such that after the cells achieve a sufficient density, oxygen limitation is achieved and subsequently maintained throughout the rest of the fermentation (e.g., dissolved oxygen less than about 10%). Agitation rate is selected to achieve a peak oxygen uptake rate (OUR) in the fermentations target 21-22 mmol/L-h.

Fermentation Conditions for Strains 1-5 and 1-8

The yeast strains 1-5 and 1-8 are run in fermenters to test succinic acid production. Fermenters are inoculated with biomass grown in defined medium (adapted from Verduyn, et. al, 1992, Yeast 8, 501-517, see Tables 1, 3, and 4). Seeds are run in 250 mL baffled flasks (50 mL working volume) at 250 rpm and 30° C. The contents of the flasks are harvested at approximately 16-24 hours incubation time with 2.5% v/v inoculum used to start fermenters. Fermenter initial working volume is 1.25 L. The cell dry weight at inoculation is found in Table 5. Fermenter media is outlined in Tables 2, 3, and 4. Carbon substrate (glucose or sucrose) is provided by the addition of 140 g/L at the start of the batch. pH is started at the ambient pH of the media (pH 4-6) and controlled at 3.5 using 28% NH₄OH until 5 mL of ammonium hydroxide solution is added to the 1.25 L batch. At this point, pH control is switched to Ca(OH)2. 1.5 g of calcium hydroxide per 100 mL deionized water is used. Once the 100 mL calcium hydroxide is exhausted pH is allowed to freefall. The fermenter systems are sparged at 0.125 slpm with air targeting 0.125 slpm aeration. Agitation rate is maintained to achieve an oxygen uptake rate of the yeast from 13-22 mmol/L-h. These fermentations are operated such that after the cells achieve a sufficient density, oxygen limitation is achieved and subsequently maintained throughout the rest of the fermentation (e.g., dissolved oxygen less than about 10%).

Dissolved oxygen is measured using Mettler Toledo INPRO® 6800 sensor (Mettler-Toledo GmbH, Urdorf, Switzerland), calibrated prior to inoculation. 0% is calibrated by sparging nitrogen, 100% is calibrated using the run conditions in the vessel as detailed above (prior to inoculation).

TABLE 1 Defined Media for Seed Flask Cultures Chemicals g/L or mL added Ammonium Sulfate 5.0 g/L Magnesium sulfate 0.5 g/L heptahydrate Potassium phosphate 3.0 g/L monobasic (MKP) Glucose 100.0 g/L Trace solution 1.0 mL Vitamin solution 1.0 mL MES buffer (0.1M) 19.0 g/L Glycerol (10% stock) 1.0 mL De-ionized Water 868 g

TABLE 2 Defined Media for Fermenters Compound Concentration (g/kg) C₆H₁₂O₆ or Sucrose 140 (NH₄)₂SO₄ 0.2 KH₂PO₄ 0.5 MgSO₄—7H₂O 0.125 Biotin Stock solution (mL) 5 1000× Trace Solution (mL) 1

TABLE 3 Trace Element 1000× Stock Solution. Chemical g/1.0 L ZnSO₄•7H₂O 4.50 MnCl₂•2H₂O 0.84 CuSO₄•5H₂O 0.30 FeSO₄•7H₂O 3.00

TABLE 4 Vitamin 1000× Stock Solution Chemical g/ 1.0 L Biotin (D-) 0.05

Cell concentration is obtained from an optical density measurement using an established conversion factor between dry cell mass and optical density. Optical density is measured at wavelength of 600 nm with a 1 cm pathlength using a model Genesys20 spectrophotometer (Thermo Scientific). Unless explicitly noted otherwise, an experimentally derived conversion factor of 1.51 OD600 units per 1 g dry cell mass is used to estimate cell dry weight (“CDW”).

Oxygen uptake rate (“OUR”) is calculated using methods known to those in the art as described above. For this example, Oxygen and CO2 values are measured by an EGAS L instrument (Sartorious). While a mass spectrometer is not necessarily used, the results obtained by the EGAS L are expected to be substantially the same. Nitrogen value is calculated as 100% less % measured CO2 minus, less % measured Oxygen. Samples are taken at whichever occurred first, 57 h batch time or the reduction of total carbon sources (glucose, fructose and/or sucrose) to <10 g/L (e.g., some batches can be sampled as soon at 33 h if the carbon sources are sufficiently exhausted at this time) and analyzed for biomass growth via OD600, succinate and glucose by high performance liquid chromatography with refractive index and ultraviolet detector.

TABLE 5 Cell Dry Weight at the beginning and end of fermentation. Initial CDW Final CDW Strain (g/L) (g/L) 1-1 0.1 6 1-2 0.1 5.9 1-3 0.1 5.5 1-4 0.1 6.8 1-5 0.2 5.7 1-8 0.1 5.2

Table 6 illustrates that a strain with a “ratio of invertase activity to glucose capacity” that is significantly less than 95 produces more succinate than a strain with a “ratio of invertase activity to glucose capacity” that is greater than 95. Accordingly, a strain having a relatively weak promoter of the invertase gene can produce more succinate than a comparable strain having a strong promoter (see also FIG. 3).

TABLE 6 Glucose consumption, invertase activity, and product formation for selected strains Ratio of invertase activity to glucose capacity (g glucose released/ (g cell dry weight * hour))/(g Invertase glucose Batch Glucose Glucose Succinate Activity consumed/ finish Consumption capacity Succinate Succinate Succinate Specific (g glucose (g cell dry time Rate (g/(g Rate (g/ Titer Yield Rate (g/ released/ weight * Strain (h) (g/(L* h)) CDW * h)) (L * h)) (g/L) (g/g) (g CDW * h)) (g CDW * h)) hour)). 1-1 56.2 2.25 0.51 1.12 60.5 0.497 0.26 No No invertase invertase present present 1-2 56.2 2.15 0.55 1.02 54.9 0.473 0.26 53.97 98.13 1-3 56.2 2.27 0.56 1.07 57.6 0.469 0.26 NM NM 1-4 56.2 2.29 0.54 1.10 57.9 0.480 0.26 9.87 18.28 NM = not measured

Table 7 illustrates that a strain expressing an invertase gene having the same promoter as strain 1-4 in either a glucose or a sucrose fermentation can achieve a succinate titer equivalent to an equivalent comparable strain without an invertase gene in a glucose fermentation. FIGS. 1 and 2 also show data supporting this conclusion.

TABLE 7 Succinate titers for glucose and sucrose fermentations of selected strains Sugar Product Sugar Sugar Specific Specific provided Consump- Consump- Rate Batch to tion tion Product Product Product (g/(g finish fermen- Rate Rate (g/(g Rate (g/ Titer Yield CDW * Strain time tation (g/(L*h)) CDW * h)) (L* h)) (g/L) (g/g) h)) 1-5 43.8 Glucose 3.17 0.76 2.44 98.8 0.770 0.58 1-8 44.2 Glucose 3.18 0.77 2.48 101.5 0.779 0.6 1-8 44.2 Sucrose 3.05 0.69 2.43 99.5 0.799 0.55

The disclosures of each and every patent, patent application, or publication cited herein are hereby incorporated by reference in their entirety. While this invention has been disclosed with reference to specific embodiments, other embodiments and variations of this invention may be devised by others skilled in the art without departing from the true spirit and scope of the invention. The appended claims are intended to be construed to include all such embodiments and variations. 

1-25. (canceled)
 26. A process for manufacturing a fermentation product comprising: fermenting a substrate using a yeast, wherein the substrate comprises sucrose and the yeast comprises an exogenous invertase gene.
 27. The process of claim 26, wherein the yeast is a yeast of the Issatchenkia orientalis/P. fermentans clade.
 28. The process of claim 27, wherein the yeast is Issatchenkia orientalis.
 29. The process of claim 26, wherein the yeast is PDC-negative.
 30. The process of claim 26, wherein the process is microaerobic.
 31. The process of claim 26, wherein the volumetric oxygen uptake rate (OUR) is 0.5 to 40 mmol O₂/(L·h). 32-34. (canceled)
 35. The process of claim 26, wherein the specific OUR is 0.2 to 13 mmol O_(2/)(g cell dry weight·h). 36-47. (canceled)
 48. The process of claim 26, wherein the fermentation substrate comprises sucrose and glucose.
 49. The process of claim 26, wherein the fermentation substrate comprises sucrose and hydrozylates of starch.
 50. The process of claim 26, wherein the fermentation substrate comprises sucrose and xylose.
 51. The process of claim 26, wherein the fermentation substrate comprises sucrose and lignocellulosic hydrozylates.
 52. The process of claim 26, wherein the fermentation substrate comprises sucrose, glucose, and xylose.
 53. The process of claim 26, wherein the process has a ratio of invertase activity to glucose consumption rate of less than
 95. 54-55. (canceled)
 56. The process of claim 26, wherein the process has a ratio of invertase activity to glucose consumption rate of at least 0.95.
 57. (canceled)
 58. The process of claim 26, wherein the fermentation yield is at least 55 percent. 59-61. (canceled)
 62. The process of claim 26, wherein the final titer is at least 30 g/liter.
 63. The process of claim 26, wherein the final titer is at least 80 g/liter.
 64. (canceled)
 65. The process of claim 26, wherein the fermentation product is selected from the group consisting of: lactic acid, citric acid, malonic acid, hydroxy butyric acid, adipic acid, lysine, keto-glutaric acid, glutaric acid, 3-hydroxy-proprionic acid, succinic acid, malic acid, fumaric acid, itaconic acid, muconic acid, methacrylic acid, and acetic acid and derivatives thereof and salts thereof. 66-68. (canceled)
 69. The process of claim 26, wherein the invertase activity is in the range of about 2.5 to 50 (g glucose released/(g CDW*h)).
 70. The process of claim 26, wherein the ratio of invertase activity to glucose consumption rate (or glucose capacity) is in the range of about 0.5 to
 25. 71. (canceled) 